During rainstorms, water that is not absorbed into the ground runs off into storm sewer systems for delivery into freshwater systems such as streams, rivers, lakes, and wetlands. While flowing across parking lots, landscaped areas, and other surfaces, the storm-water runoff picks up debris and pollutants and carries them into the storm sewer systems. Particularly large amounts of pollutants are picked up at shopping centers with large parking lots, oil-change and auto-repair shops, gas stations, and so forth. These pollutants include motor oil and other hydrocarbons, particulate matter such as sand and grit, and miscellaneous debris such as vegetative matter, paper, plastic, and foam cups. For example, about 200 pounds of miscellaneous debris and 500 pounds of sand and grit is commonly carried off by storm-water runoff from some one-acre parking lots in 90 days.
To maintain freshwater systems, most cities and counties have regulations requiring that some of the pollutants be removed from the storm-water runoff before entering their storm sewer systems. In order to meet these regulations, facilities typically install on-site pollution traps to filter the storm-water runoff. These pollution traps are sometimes referred to as “oil/grit separators.”
Most conventional pollution traps provide only “first flush” filtration during the typical local storm event, but permit bypassing the filtration stage for larger storms. In fact, many jurisdictions require bypassing, some even at typical storm water flows. Bypassing filtration is a problem because most pollutants are more easily picked up and transported by storm water during higher flow periods. Unfortunately, just when the traps are needed most, a lot of pollutants bypass them and are delivered into the storm sewer systems. And most pollution traps that do not provide for bypassing accommodate the larger flows because they are oversized, which adds significantly to the cost to build, install, and maintain them.
Ground water run-off frequently carries petroleum products with it. Such pollutants include oil, gasoline, greases, etc. these petroleum pollutants can originate in streets, parking lots, service garages, etc. It is important to try to eliminate these pollutants from the water run-off before it reaches the water table. One physical property of these pollutants that can be exploited in trying to separate them from the water run-off is that most of them float (i.e., they are less dense than water). Prior efforts to remove such petroleum pollutants from water run-off have been less than completely successful.
Accordingly, it can be seen that a need remains for a pollution trap is effective for removing oil, gasoline, etc., from water run-off. Furthermore, a need exists for such a pollution trap that is cost-efficient to build, install, and maintain. It is to the provision of a pollution trap meeting these and other needs that the present invention is primarily directed.